The term "standard resistance" or "standard running resistance" is herein used to mean any force which opposes the motion of an automotive vehicle which is driven to keep rolling over the surface of a flat road having 0% gradient at a constant vehicle speed. The term "running resistance" is herein used to mean any force which opposes the motion of an automotive vehicle which is driven to keep rolling over the surface of a road at a constant vehicle speed. Running resistance is equal to standard resistance if an automotive vehicle is driven to keep rolling over the surface of a flat road having 0% gradient at a constant vehicle speed. Running resistance increases and becomes greater than standard resistance if the automotive vehicle is accelerated to increase speed from the constant vehicle speed. The term "acceleration resistance" is herein used to mean this increment or difference in running resistance that has occurred due to acceleration. Running resistance is greater when the automotive vehicle is driven to keep rolling over the surface of a flat road having gradient greater than 0% at a constant vehicle speed than standard resistance for the same vehicle speed. The term "gradient resistance" is used to mean this increment or difference in running resistance.
JP-A 9-242862 discloses a vehicle control system in which a speed ratio between an input shaft and an output shaft of an automatic transmission is controlled in response to road gradient, throttle opening degree, and vehicle speed. In order to estimate road gradient of a road, over which the vehicle is rolling, a road gradient torque (T.alpha.) is determined by subtracting from a driving torque (To) a sum of a flat road running resistance torque (Tr) and an acceleration resistance torque (T.alpha.). A characteristic of variation of flat road running resistance torque (Tr) against variation of vehicle speed is mapped. This mapped data are retrieved using a current reading point of vehicle speed to give a value of flat road running resistance torque (Tr).
JP-A 9-286261 teaches combining a traction control system with an anti-lock brake system. In this case, operation of the traction control system is suspended when estimated vehicle speed drops below a predetermined value during deceleration to avoid coincidence of the two control systems.
JP-A 10-266882 teaches using a map to accomplish smooth variation in driving force with variation in throttle opening degree for a small throttle opening degrees falling in a small range. The map contains various target values in driving force against various values in throttle opening degree. The setting is such that, for the small range of throttle opening degrees, a change in the target values in driving force against a change in throttle opening degree is small.
In the case where the estimated output torque and vehicle speed are used in determining gradient resistance, the calculation accuracy of the gradient resistance depends on the detection accuracy of the vehicle speed. However, during operation at low vehicle speeds, the detection accuracy of the vehicle speed becomes very low due to low revolution speeds of the drive shaft at low vehicle speeds. Thus, high calculation accuracy of the gradient resistance is not expected during operation at low vehicle speeds, allowing overestimation of driving force at low vehicle speeds as the vehicle move off from a standstill as shown in FIG. 11. This overestimation causes an excessive increase in correction of driving force, deteriorating ride feel of the vehicle operator.
JP-A 8-219242 discloses a driving force control system wherein a vehicle speed sensor detects revolution speed of a transmission output shaft and a change with respect to time of varying vehicle speed is calculated to give a vehicle acceleration. The vehicle acceleration is used to calculate gradient resistance as well as rolling resistance, air resistance and drive torque. The calculated gradient resistance plays an important role in varying ratio chance characteristic to provide driving force to meet varying road gradient. During operation at low vehicle speeds, a low level of detection accuracy of vehicle speed may cause an undesired change in driving force.
An object of the present invention is to provide a driving force control that prevents occurrence of undesired change in driving force during operation at low vehicle speeds.